The invention relates to the field of ultrasonic measurements, and more particularly to a method and apparatus for ultrasonic measurement of a rate of flow.
The invention is applicable to the measurement of the rate of flow of petrol and its products, chemical products, food products, and water in melioration systems.
The term "flowmeter" as used in this text applies to apparatus designed to measure both the velocity of flow and the rate of flow, since measurements based on acoustic waves deal with the velocity of flow which is a function of the rate of flow.
When a flow running, for example, through a pipeline, is to be measured, it is desirable that its movement is not disturbed during measurement. Another requirement is concerned with the availability of a relatively cheap apparatus which can provide highly accurate measurements and long service life and which cannot be influenced by the temperature and other changes of the physical properties of the medium under control.
Taken as a first approximation are the results provided by ultrasonic frequency-pulse measurement methods in which the influence of the physical properties of the medium under control is eliminated. These methods are realized by ultrasonic flowmeters based on a synchronization ring-like arrangement which constitutes a pulsed generating system with a delayed acoustic feedback. Such an arrangement is operated in a pulse auto-circulation mode.
To measure the rate of flow, use is made of apparatus with one or two acoustic channels, in which an acoustic channel is a space through which it is intended to pass the medium under control and to separate two electroacoustic transducers.
Known in the art is a method of ultrasonic measurement of the rate of flow, which deals with the passage through the medium under control of two auto-circulation pulse trains which in turn pass through two respective acoustic channels. The pulses in the acoustic channels are in downstream and upstream direction, respectively. The difference between the repetition rates of the pulses of the two pulse trains, may be used to determine the rate of flow.
A known ultrasonic flowmeter designed to carry out the aforedescribed method utilizes two synchronization ring-like arrangements, each of which is provided with a an amplifier-former, a generator producing the arrangement signals, and an excitation pulse former connected in series. The series connections are coupled to two respective electro-acoustic transducers which are separated by a space, through which the medium under control is passed, and having a relative orientation allowing for the transmission and reception acoustic signals passing between them in a direction which makes an angle different from 90.degree. with the direction in which the medium under control moves. The flowmeter also comprises frequency multipliers and a measuring unit (cf. U.S. Pat. No. 3,625,057, Int. Cl. G01P 5/00).
In each of the synchronization ring-like arrangements, the generator pulse is applied to the excitation pulse former, whose output produces a singal which is applied to the respective electroacoustic transducer, which emits a pulse into the medium under control. After the other transducer has received that pulse, said pulse is applied again to the corresponding excitation pulse former and the pulses are subject to an auto-circulation process in the arrangement. The frequency of each ring-like arrangement is multiplied in the corresponding frequency multipliers and the measuring unit is then operated to determine the frequency difference, which is a measure of the rate of flow of the medium under control.
Since two acoustic channels are used in the aforedescribed flowmeter, an extra error occurs due to a difference in the paths covered by the pulses. This difference is not caused by the properties of the medium.
To provide for higher accuracy of measurement, the described flowmeter should be fabricated and assembled with a great degree of precision. The temperature condition of the electronic circuitry and the pipeline section being measured must be held constant within very close limits. The lengths of the acoustic channels must be held constant with an accuracy of several micron units. Finally, the error due to the difference between the paths of the pulses, not caused by the properties of the medium, should be compensated. All of these disadvantages place a limitation on the possible uses of the flowmeter in systems requiring no higher accuracy of measurement.
There is another method of ultrasonic measurement of the rate of flow, which is free from error due to nonidentical condition of the channels. This method comprises the steps of alternate passage through the medium under control and through a single acoustic channel of two auto-circulation pulse trains of opposite directions, storing, for the time of passage of the pulses of one direction, the repetition rate of the pulses propagating in the opposite direction, and determining the rate of flow using the difference between the repetition rates of the two pulse trains.
The flowmeter for realizing the aforedescribed method comprises two electroacoustic transducers, a one-channel measuring circuit based on a synchronization ring-like arrangement with a storage device, and a measuring unit (cf. the USSR Inventor's Certificate No. 191,155, Int. Cl. G01f).
The ring-like arrangement of the flowmeter is operated alternately in the downstream and upstream mode. The storage device is used to store, for the time of passage of the pulses of one direction, the repetition rate of the pulses of the opposite direction. The measuring unit uses the difference between the stored and the present value of the repetition rates of the two auto-circulation pulse trains to indicate the rate of flow.
In this method, alternate passage through the medium under control of two auto-circulation pulse trains of opposite directions results in a condition where ultrasonic waves pass through sections of the medium which have different physical properties which are changed between the switching cycles. This causes an extra error of measurement of the rate of flow. In addition, the method is not applicable to measuring the rate of pulsating flows, since the measurement process has a large time constant. To attain higher accuracy of measurement, the flowmeter must be provided with a re-adjustable storage device which could store the frequency with an error of the order of 10.sup.-9. This is a feature attainable at present with extremely great difficulty.
A method of ultrasonic measurement of the rate of flow known in the art comprises the steps of concurrent passage through the medium under control and through a single acoustic channel of two auto-circulation pulse trains of opposite directions, eliminating the instants at which the pulses of the two pulses train are brought into coincidence, and determining the rate of flow using the difference between the repetition rate of the pulses of the two trains. According to the method, the elimination of the instants when the pulses of the trains are brought into coincidence is attained by relative shifting in time of each of the two pulse trains. The number of these shifts, characteristic in indirect way of the difference between the repetition rates of the pulses of the trains, is a measure of the rate of flow.
The flowmeter for realizing th aforedescribed method comprises two synchronization ring-like arrangements which utilize a single acoustic channel, two units adapted to shift the autocirculation pulse trains, a flip-flop adapted to count the number of shifts, and a frequency meter to measure the repetition rate of the pulses of the flip-flop, which is an indirect measure of the rate of flow (cf. the USSR Inventor's Certificate No. 479,000, Int. Cl. G01F 1/00).
The method is disadvantageous, because the number of shifts of the pulse trains differs from the true value of the difference between the arrangement frequencies, which increases the error of measurement of the rate of flow. In addition, the method requires that the pulse trains be shifted in time in strictly similar manner, since this influences the accuracy with which the rate of flow is measured. However, it is very difficult to meet the last-mentioned requirement.
The described flowmeter described features an intrinsic methodical error of measurement of the rate of flow. In addition, the units designed to shift the pulse trains as well as the elements of the ring-like arrangements are allowed to have a time non-identity of the order of several nanosecond units. This requirement makes the flowmeter barely realizable, so that the advantages of the single-channel method vanish. The flowmeter measures the rate of flow slowly. For example, with the pipeline diameter equal to 1 m and with a flow velocity of 0.1 m/s. the difference between the ring-like arrangements will be about 0.1 Hz, which means that too large a measurement time of 10 s is required. In addition, it is impossible to automatically drive the flowmeter into the auto-circulation mode or to automatically restore its operation after a temporary disturbance of the acoustic channel, which may be caused by ultrasonic scattering relating to gas bubbles or foreign matter in the media under control. This, of course, places certain limitations on its uses in automatic control systems.
There is an ultrasonic flowmeter comprising two synchronization ring-like arrangements which include, respectively, series-connected inhibitors, excitation pulse formers, and electroacoustic transducers and an amplifier-former, all of which are common to the two synchronization ring-like arrangements. The transducers are separated from each other by a space, through which the medium under control is passed, and have a relative orientation allowing for the transmission and reception of acoustic signals passing between them in a direction which makes an angle different from 90.degree. with the direction in which the medium under control moves. The flowmeter comprises trigger pulse units coupled to the corresponding synchronization ring-like arrangements and having, respectively, self-excited oscillators whose inputs are connected to outputs of search/automatic phase control networks, and whose outputs are connected, via corresponding frequency dividers, to inputs of AND gates, to inputs of the search/automatic phase control networks, and to inputs of storage elements whch have their outputs coupled to the other inputs of the search/automatic phase control networks and the AND gates. The flowmeter further comprises a measuring unit common to the two synchronization ring-like arrangements and having its inputs coupled to outputs of the self-excited oscillators (cf. USSR application for U.S. patent, Ser. No. 016,339, filed Feb. 28, 1979 and allowed Apr. 16, 1980 as Pat. No. 4,240,292.
The respective trigger pulse unit operates to drive the corresponding ring-like arrangement into an auto-circulation pulse mode and is then switched off. The trigger pulses are applied again to the ring-like arrangement when the acoustic channel is disturbed due to ultrasonic scattering relating to gas bubbles or foreign matter in the medium under control.
The flowmeter is automatically triggered and restores its operation after an occurrence of a temporary disturbance of the acoustic channel. The flowmeter has high operational speed and good noise immunity.
The disadvantages of the flowmeter are that the two ring-like arrangements can operate steadily in a single acoustic channel in a sequential mode only. This reduces the accuracy of measurement of the rate of flow, since the physical properties of the medium under control tend to vary between the switching cycles. In addition, the frequency of one ring-like arrangement should be stored during operation of the other, which causes an extra measurement error.